In image forming apparatuses, such as electrophotographic copying machines, facsimile machines, and laser beam printers, a thermal fixing method has been generally employed in which, in the final stage of printing/copying, a fixing belt (i.e., a fixing sleeve, fixing tube roller, or the like) provided with a heating source inside and a pressure roller are pressed into contact with each other, and a transfer-receiving body onto which a toner image has been transferred is passed therebetween, whereby unfixed toner is melted by heating.
As the fixing belt, there has been generally used a fixing belt having a structure in which a resin layer having excellent elasticity, releasability, wear resistance, and the like is disposed on a surface (surface to be in contact with a transfer-receiving body) of a tubular base member composed of a high-strength, heat-resistant resin, such as polyimide, or a fixing roller including a cylindrical base member composed of metal or polyimide and a resin layer having excellent elasticity, releasability, wear resistance, and the like disposed on the outer circumferential side of the base member. As the resin layer having excellent elasticity, releasability, wear resistance, and the like, a fluororesin coating layer has been widely used.
In order to obtain a good fixing property, it is necessary that a transfer-receiving body be sufficiently heated by a heating source provided inside a fixing belt. Consequently, the fixing belt is required to have excellent thermal conductivity.
Furthermore, in the fixing process of an image forming apparatus using a plurality of kinds of color toners, it is necessary to mix a plurality of kinds of color toners in a molten state when fixing is performed. Consequently, a fixing belt used for this purpose is required to have a proper degree of elasticity such that color toners are sufficiently enveloped so as to be melted and mixed.
As described above, since the fixing belt is required to have excellent thermal conductivity and a proper degree of elasticity, in order to satisfy these requirements, PTL 1 proposes a fixing belt including a heat-resistant elastomer layer (elastic layer) provided on the outer circumferential side of a tubular base member, and a fluororesin layer provided further thereon, in which the thickness of each of the tubular base member, the fluororesin layer, and the heat-resistant elastomer layer is specified, and the relationships between thickness, hardness, and thermal conductivity of the heat-resistant elastomer layer are defined so as to be within specific ranges (Claim 2). In order to satisfy these conditions, a technique is proposed in which an inorganic filler that improves thermal conductivity, such as silica, alumina, or boron nitride, is compounded into the heat-resistant elastomer (paragraph 0015).
Furthermore, PTL 2 discloses a fixing belt having a laminated structure in which a heat-resistant elastomer layer is disposed on the surface of a metal tube or heat-resistant plastic tube, and a silicone rubber or fluororesin layer is further disposed on the outer surface thereof, in which the deformation under load and the thickness of each of the layers are within predetermined ranges, and furthermore, the hardness and thermal conductivity of the heat-resistant elastomer layer are within predetermined ranges (Claim 1).
A method is also proposed in which, in order to set the thermal conductivity of the heat-resistant elastomer layer within the predetermined range, an inorganic filler that improves thermal conductivity, such as silica, alumina, or boron nitride, is compounded thereinto, and it is disclosed that by this method, heat from a heating source can be quickly supplied to the outer surface of the fixing belt (paragraph 0012).